This invention relates to an X-ray apparatus utilizing a rotary anode type X-ray tube, more particularly a rotary anode type X-ray tube using high acceleration voltage.
Radiography is now widely used to examine metal structure, casting, or a product. In order to obtain a clear radiograph of a nuclear fuel rod comprising a casing and a fissionable material contained therein, it is necessary to use hard X-rays having a high transmittivity for such substance. Such hard X-rays can be produced by impressing a high acceleration voltage across an anode electrode that is a target generating X-rays and a cathode electrode that emits an electron beam. Although a stationary anode type X-ray tube can operate under a considerably high acceleration voltage, in the case of a rotary anode type X-ray tube, about 150 KV is the upper limit of the acceleration voltage from the standpoint of electrical insulation.
Where a nuclear fuel rod is examined by X-rays, radiations from the fissionable substance, uranium for example, are sensed by X-ray films so that if the exposure is made for a long time, the background density of the exposed X-ray film would increase, thus erasing an image necessary for the inspection. For this reason, a short time exposure is essential. To obtain a radiograph of the desired contrast by a short time exposure, it is necessary to generate a large amount of X-rays per unit time and hence to greatly increase the tube current. In the case of the stationary anode type X-ray tube in which a definite point or small area on the target is subjected to the bombardment of the electron beam, the anode electrode can not withstand large tube current thus damaged. On the other hand, the rotary anode type X-ray tube in which the impinging point on the targent moves constantly can withstand such large current. For example, comparing the inputs (acceleration voltage X tube current) of a conventional rotary anode type X-ray tube and a stationary anode type X-ray tube, the input of the latter is at most 4KW while the former can produce an input up to 100KW. Assume now that both types utilize the same acceleration voltage for producing X-rays of the same hardness, the rotary anode type X-ray tube can produce tube current 25 times that of the stationary anode type X-ray tube thereby greatly increasing the quantity of X-rays generated per unit time.
In this manner, although the rotary anode type X-ray tube is suitable to operate delivering a high output, there is a limit for the acceleration voltage in view of the problem of electrical insulation as above described.
More particularly, since the prior art rotary anode type X-ray tube has been energized by a high voltage circuit with its neutral point grounded, a positive high potential is impressed upon the anode electrode whereas a small potential difference at low level voltage is impressed across a stator disposed about a rotor for driving the anode electrode. For this reason, the insulation between the anode electrode and the stator should be high and poor insulation renders inoperative the X-ray apparatus utilizing the X-ray tube. A cylindrical insulator generally made of a plastic or glass is used as the insulator between the anode electrode and the stator. When operated under anode potential, such insulator is liable to breakdown. Such difficulty can be avoided by increasing the gap between the anode electrode and the stator as by using a thicker insulating cylinder. Increased gap renders it difficult to transmit magnetic flux from the stator to the rotor thus decreasing the efficiency of the motor with the result that the number of revolutions of the anode electrode is decreased or the starting time of the anode electrode is increased. Increase in the width of the gap also increases the size of the X-ray tube unit.